1. Field of the Invention
The present invention relates to an image forming apparatus which conveys a sheet to an image recording device to record an image and, more particularly, to an image forming apparatus which can suppress skew feeding of a sheet in image transferring.
2. Description of the Related Art
In an electrophotographic image forming apparatus, after a toner image is formed on an image bearing member, a transfer portion brings a sheet into contact with the image bearing member to electrically transfer the toner image onto the sheet. At this time, a method using a transfer roller which transfers the image while bringing the sheet in stable contact with the image bearing member and also stably conveys the sheet is popularly used.
In a conventional electrophotographic image forming apparatus, in order to improve the printing accuracy of a toner image on a sheet, the following configuration is employed.
A first conventional image forming apparatus is shown in FIG. 8. This image forming apparatus 100 feeds a sheet P supported on a sheet stacking portion 120 with a feed roller 124. The fed sheet P is fed to a stopped registration roller pair 123 through a conveyance roller pair 122. The distal end of the sheet P is brought into contact with a registration roller pair 123 and further pushed into the registration roller pair 123 to form a loop, and the skew of the distal end is corrected to perform alignment (see FIGS. 9A and 9B). Thereafter, the registration roller pair 123 is rotated to convey the sheet P to the transfer portion. The transfer portion transfers the toner image from a transfer belt (an image bearing member) 131 serving as an image bearing member while being conveyed by both the registration roller pair 123 and the transfer roller 135. The sheet P transferred onto the toner image is transferred is conveyed to a fixing device 144 by the drive of the transfer roller 135.
Therefore, the sheet P is subjected to a process of sheet alignment correction by the registration roller pair 123, printing accuracy such as a distal-end registration roller accuracy and a perpendicularity are preferably obtained when the toner image is transferred. As a reference in which such a configuration is disclosed, Japanese Patent Application Laid-open (JP-A) No. 1-53886 is known.
Not only correction of the distal end of the sheet by the registration roller pair 123 but also correction of a position in a sheet width direction which is perpendicular to a sheet conveyance direction can be made possible by arranging a skew roller on the upstream side of the registration roller pair 123. Such an apparatus, as shown in FIGS. 10A to 10D, a reference wall 223 is arranged on a part of a sheet guide near the skew roller 226 in parallel to the conveyance direction of the sheet. The skew roller 226 causes the sheet-side end to extend from the reference wall 223 to convey sheet along the reference wall 223 (see FIG. 10A) . Furthermore, a registration roller pair 227 which moves the sheet P to a conveyance center 228 (reference position at a center in the sheet width direction) reciprocally operates (reciprocal operation in the vertical direction in FIG. 10A). In this manner, the sheet center 229 (center of the sheet in the sheet width direction) is matched with the conveyance center 228 to convey the sheet to the transfer portion (see FIGS. 10A to 10D).
In the transfer portion, the sheet P is conveyed by both the registration roller pair 227 and a transfer roller 235, and a toner image is transferred from a image bearing member 231 onto the sheet P (see FIG. 10C). The sheet P on which the toner image is transferred is conveyed to a fixing device 244 by the transfer roller 235. At this time, in order to cause the registration roller pair 227 to perform a reciprocal operation for a next sheet P′ immediately after the distal end of the sheet P is held by the nip of the transfer portion, the registration roller pair 227 performs a reciprocal operation which returns the sheet P to an original standby position in a state in which the nipping of the registration roller pair 227 is released (see FIG. 10D).
In this manner, the sheet P is subjected to a process of sheet alignment correction to preferably obtain printing accuracy such as a lateral registration (sheet width direction registration) accuracy and a perpendicularity when the toner image is transferred. As a reference in which the configuration is disclosed, JP-A No. 11-189355.
However, the above conventional technique has the following problem.
As described in JP-A No. 1-53889, in the image forming apparatus which corrects sheet alignment to an image by the registration roller pair 123, the sheet P is conveyed to the transfer portion. Thereafter, after the sheet P is removed from the nip of the registration roller pair 123, the sheet P must be solely conveyed depending on the size of the sheet P.
In this case, when the outer diameter of the transfer roller 135 is not uniform, or when the degree of parallelization to the registration roller pair 123 is low, skew feeding occurs such that the sheet P is skewed. When an image ratio of a toner image to be transferred extremely deviates from a direction perpendicular to the sheet conveyance direction, a friction coefficient between the transfer belt 131 and the sheet P deviates. More specifically, when the friction coefficient decreases, a transfer nip portion serving as a portion where the toner image ratio is high has a conveyance power lower than a portion where the image ratio is low. In this manner, the uniformity of the conveyance power skews the sheet conveyed by a single transfer roller. The skew feeding of the sheet disadvantageously deteriorates an image position accuracy to degrade image quality.
Similarly, in the image forming apparatus disclosed in JP-A No. 11-189355, even though a lateral registration accuracy of the sheet P for a toner image is secured by the skew roller 226 and the reference wall 223, the respective rollers of the registration roller pair 227 are separated from each other. For this reason, the sheet is conveyed by the single transfer roller in a state in which the nip of the sheet P is released. For this reason, the sheet is skewed.
In this manner, the image position accuracy is deteriorated to cause deterioration of image quality. In addition, in the image forming apparatus described in JP-A No. 11-189355, since the registration roller pair 227 releases their nip before the registration roller pair 227 passes through the rear end of the sheet P, a distance of conveyance singularly performed by a transfer roller becomes long to further increase an amount of skew feeding. Furthermore, in order to avoid this, a reciprocal operation to a standby position may be performed after the rear end of the sheet P passes through the registration roller pair 227. However, since a sheet conveyance interval of operation time for returning the registration roller pair 227 to the standby position must be assured, productivity is considerably deteriorated.